I have a conventional wood-burning fireplace made of cement block and brick. The brick extends about 20 inches away from the edge of either side of the firebox. Next to the edge of this brick is a wall made of sheetrock that extends another six feet. How close can an electrical box be placed in the sheetrock wall next to the brick? Is there a code that governs this condition? Thank you.

There is no code restriction. It can be right up against the brick, if you like.